An antibody-drug conjugate (ADC) composed of a monoclonal antibody against the immunoregulatory protein B7-homologue 3 (B7-H3, CD276) conjugated, via a solubilizing linker, to a B-cell lymphoma extra long (Bcl-XL) inhibitor, with potential antineoplastic activity. Upon administration of mirzotamab clezutoclax, the anti-B7-H3 monoclonal antibody moiety targets and binds to B7-H3 expressed on tumor cells. Upon binding, internalization and linker cleavage, the Bcl-XL inhibitor targets, binds to and inhibits the activity of the pro-survival protein Bcl-XL. This restores apoptotic processes and inhibits the proliferation of B7-H3-expressing tumor cells. B7-H3, a type I transmembrane protein and a member of the B7 co-stimulatory protein superfamily, is overexpressed on certain tumor cell types and on various immune cells. It plays a key role in tumor growth and immune responses.